1. Field of the Invention
The invention relates generally to techniques for calibration of pulsed neutron logging measurements.
2. Background Art
Oil and gas production companies typically want to produce as much hydrocarbon as possible in each down-hole drilling operation. Quite often a well contains recoverable quantities of hydrocarbon gas in formations are bypassed during drilling. More accurate determination and interpretation of the measurement data in these reservoirs would help the operator decide whether production of these reservoirs is justified.
A major component of the interpretation of hydrocarbon content in a formation is to determine water saturation in the formation. Water saturation is the fraction of water in a given pore space. The remainder in the pore space would be hydrocarbon. Neutron and gamma ray logging is commonly used technique for determining formation pore spaces and water saturation. Neutron and gamma ray logging may be performed using different neutron tools, including pulsed neutron tools.
Pulsed neutron capture (PNC) tools have been used for years to evaluate the presence of hydrocarbon in formations. PNC tools generates high energy neutrons that interact with surrounding atoms to produce energy in at least two ways. First, high-energy neutrons collide “inelastically” with nuclei in the formations, exciting the nuclei and causing the nuclei to release gamma rays. Second, these high energy neutrons eventually lose enough energies to reach “thermal neutron” state. Thermal neutrons can be “captured” by nuclei in the formations. Upon capture of thermal neutrons, the nuclei become excited and would release gamma rays when they return to the ground states. Such released gamma rays may be detected by gamma ray detectors to provide a measure of the degrees of thermal neutron captures.
Most PNC tools measure the thermal neutron capture characteristics, or macroscopic capture cross-section (“sigma” or “Σ”), of a formation by detecting and counting gamma rays of the second type, i.e., those that occur as a result of thermal neutron capture. The thermal neutron capture tools would monitor the emitted gamma rays over a given period of time after a burst of high energy neutrons. Because hydrocarbons generally have low sigma values, the presence of hydrocarbons in a formation would exhibit long neutron capture times (i.e., low capture efficiency). Although pure water also has a low sigma, formation water typically contains salts that would increase sigma values of the formation water. For example, chlorine, usually in the form of salt (NaCl) in formation water, has a large sigma value, as compared to other elements found in the formation. Formation connate water may contain salts at various concentrations, which would result in different sigma values. Therefore, accurate water saturation (Sw) based on formation sigma determination would depend on accurate determination of water salinity.